Polytetrafluoroethylene (hereinafter “PTFE”) is used for various types of durable material due to its high melting point (approximately 330° C.), and excellent chemical stability and chemical resistance. Moreover, as it also has excellent weather resistance and water repellency, it is used as structural film material for dome roofs and the like. As its melt viscosity is very high, it is difficult to mold PTFE into film by melt molding such as extrusion molding. Conventionally, PTFE film is manufactured by the skiving technique in which film is shaved by the “katsuramuki” (rotary cutting) procedure—i.e., by shaving into thin-layered film form—from a block-shaped precursor obtained by sintering PTFE powder.
On the other hand, as a method for enhancing film strength, a method is known in which film that has once been molded is stretched to extend and orient the macromolecular chain forming the obtained film. However, with respect to PTFE film manufactured by the skiving technique, as the film breaks when it is stretched even at low stretching ratio, it has been difficult to sufficiently enhance strength by stretching.
As methods for obtaining high-strength PTFE film by stretching, for example, methods utilizing solid-state stretching have been disclosed in which stretching is conducted below the PTFE melting point, i.e., in a state where the PTFE is substantially unmelted. Specifically, the following methods (i) to (iii) may be cited.                (i) A method in which film obtained by compression molding of PTFE powder at a temperature below the melting point thereof is stretched at a temperature where the obtained film does not substantially melt (Patent Document 1).        (ii) A method in which PTFE powder to which lubricant has been added is extrusion molded and rolled to obtain a sheet, and porous film obtained from the said sheet by removal of the aforementioned lubricant is stretched at a temperature under the PTFE melting point, followed by being baked until it becomes non-porous (Patent Document 2).        (iii) A method in which PTFE powder to which lubricant has been added is extrusion molded, and subjected to biaxial stretching below the PTFE melting point (Patent Document 3).        On the other hand, the present inventors hereby report the following method (iv).            (iv) A method in which stretched film is obtained by melt stretching wherein PTFE film is stretched in a melted state (Non-Patent Documents 1 and 2). This focuses attention on the fact that PTFE has remarkably high viscosity at temperatures above the melting point, and attempts to conduct melt stretching that is infeasible with ordinary thermoplastic resin.